Hide size class computation behind a layer of indirection.

This class removes almost all the dependencies on size_classes.h, accessing the
data there only via the new module sc.h, which does not depend on any
configuration options.

In a subsequent commit, we'll remove the configure-time size class computations,
doing them at boot time, instead.

test/unit/slab.c

@@ -3,7 +3,7 @@
 TEST_BEGIN(test_arena_slab_regind) {
 	szind_t binind;
 
-	for (binind = 0; binind < NBINS; binind++) {
+	for (binind = 0; binind < SC_NBINS; binind++) {
 		size_t regind;
 		extent_t slab;
 		const bin_info_t *bin_info = &bin_infos[binind];